Steam cooking ovens use steam to cook food contained in a cooking cavity. Typically, electrical resistance heating elements are used to heat water in a water reservoir to generate steam. While there are many configurations of steamers, one approach is to locate the reservoir inside the cooking cavity. That is, water usually is contained in a dedicated reservoir in the bottom of the cooking cavity. In such ovens, one or more heating elements are either immersed in the water or enclosed within heating blocks located immediately below the reservoir.
When heating elements are directly immersed in the steam generating water, one of the common problems is that, since the heating element is in direct contact with the water, any impurities in the water will tend to coat the element (i.e.: calcify) and thereby reduce the heating element's efficiency and capability. In addition, the heating element must be suspended above the bottom surface of the reservoir to avoid heating the reservoir housing. Because of this, it is possible for the water level to fall to a point where the heating element is exposed to the atmosphere within the cooking cavity. When this occurs, there is a high risk of damaging the heating elements despite the protections provided by temperature limiting switches.
In an alternate configuration, a heating element is encased in a rigid thermally conductive heating block and the heating block is in turn attached to the underside of the cooking cavity. The heating block helps diffuse heat more evenly across the bottom surface of the cooking cavity producing more even cooking. Enclosing heating elements in heating blocks also increases the thermal mass surrounding the heating element, leading to more stable and precise control of the heating element itself. Further, physically separating the heating elements from the water reduces calcification problems.
One problem with using heating blocks is that the bottom surface of the reservoir will rarely be fully coplanar with the contact surface of the heating block. Casting or machining irregularities during production and warpage of the reservoir floor during use are factors which contribute to this problem. Uneven contact between the heating block and the reservoir floor leads to hotspots and uneven cooking. In extreme cases, the bottom of the reservoir floor may warp to such an extent that localized areas of the bottom surface protrude above the water level, creating more severe hotspots. One method of addressing this problem is to machine the surfaces to very close tolerances. However, this approach is expensive and does not address changes in shape that may occur during repeated use.
Thus, there remains a need for an economical way to compensate for surface irregularities and mis-matches between heating blocks and reservoir surfaces in steam ovens.